Troubleshooting Trailer Brakes That Engage with Direct 12V Power But Not with Brake Controller

Updated 04/11/2019 | Published 04/10/2019

Products Featured in This Question

etrailer Electric Trailer Brakes - 10" - Left/Right Hand Assemblies - 3,500 lbs

(698)

$126.47

Digital Multimeter with 42" Long Test Leads - LED Display

(3)

$22.17

Question:

I just put new brakes on my trailer when I apply the brake controller I can hear the brakes hum but theyre not engaging Ive adjusted them up so that they slightly drag they still dont engage if I hook a jump box directly to the brakes they lock up I put a test light on the power wire at the brakes and had someone adjust the power setting on my brake controller and the test light got bright and dim so I know that the brake controllers working

asked by: Floyd G

Expert Reply:

Thank you for the details on your testing of your new trailer brakes # AKEBRK-35.

Your successful jump box test direct to the brake circuit on the trailer's 7-way plug tells us the trailer side of things is working (since the brakes will lock up with that full power input) so this suggests the problem is on the vehicle side. The brake pressure should be adjusted so that when you spin a wheel by hand you do feel some resistance.

If you happened to test each individual brake, rather than going through the 7-way plug, then it is still possible there could be a problem in that part of the trailer wiring, but if you tested at the 7-way trailer plug it is safe to conclude the trailer is ok.

I suggest using a voltmeter like # PT89ZR at your vehicle 7-way socket. Set the controller for maximum power and maximum boost and have a helper press the manual control while you apply the meter probe to the brake circuit contact at the 5:00 position. With full power and full boost your meter should read close to 12.8V. If not then you probably have corrosion within the 7-way or damaged wiring leading to it.

You can repeat the same voltmeter test right at the back of the controller. This will potentially save you the time and effort of troubleshooting the vehicle wiring.

Cut the blue wire a few inches from the controller so you can re-connect it after the test. Apply the meter to the output wire in the same way you did at the 7-way. If you get full power at the controller's output wire then you know the issue is in the wiring between the controller and the 7-way socket. If you still don't get full power then the controller is damaged internally.